Static secret for Travis ENV variables

Fixes #5 - Travix will not resolve `secure` parameters on build jobs resulting from pull requests. This is pretty essential for our GitHub based workflow.

Also, since the value is encrypted with the repository's public key the build would always fail on any forks. Forked repo's have their own public keys.

Resolution required setting a static `SECRET_TOKEN` value in the .travis.yml file without the Travis `secure` encryption. This only affects ephemeral builds in the Travis test environment. It won't become a default value for builds outside of Travis.